The efficient cooling of thermally highly loaded components of a gas turbine is an indispensable condition for operating modern machines. There have accordingly been rapid developments in cooling methods.
The simplest form of cooling is convection cooling, in which a cooling medium flows over one surface of a component and extracts heat from the latter, whilst another surface is subjected to the introduction of heat. One disadvantage of convection cooling is, in particular, that the entire heat to be discharged has to be transported through the component wall. The surface subjected to the introduction of heat is at a substantially higher temperature than the cooled surface. Moreover, considerable temperature gradients through component walls and therefore also thermal stresses are caused.
Film cooling, known from U.S. Pat. No. 3,527,543 has therefore been preferred for a long time, in which a coolant--preferably air extracted from the compressor or steam--flows through the component wall from a cold-gas side to the hot-gas side subjected to hot gas. In this case, on the one hand, the coolant absorbs heat from the material while it is flowing through the blow-out orifices. On the other hand, a film of relatively cool medium is laid over the hot-gas side of the component and protects this side from direct contact with the hot medium. However, if film cooling is adopted completely in modern gas turbines, the coolant consumption rises excessively.
Development has therefore tended, to an increased extent, toward using the coolant prior to blow-out for efficient convection cooling. In impact cooling, which may be gathered, from DE 44 30 302, for example, the coolant impinges at as high a velocity as possible onto the component to be cooled, thereby intensifying the convective transmission of heat from the component to the coolant. However, the good cooling effect is achieved at the expense of comparatively high pressure losses on the cooling side.
Furthermore, impact cooling cannot readily be used everywhere. Problems arise particularly in the region of the blade trailing edges due to the geometric design. On the other hand, it is precisely there where good cooling of the material is necessary, since, in comparison with the material thickness, the surface exposed to the hot gas is large, whereas the surface of the cold-gas side is relatively small. Furthermore, it has been shown, in practice, that there are, in fact, major problems, in a closed structure, in achieving a flow through such narrow gaps as are present on the cooling side in the region of the trailing edge. In this region, the inner walls of the hollow component butt against one another at an acute angle, and the flow boundary layers on the cooling side coalesce. The flow velocity in the narrow gap becomes very low, and the coolant flow is displaced into other parts of the component interior. The displacement action of the flow boundary layers thus place narrow limits on any improvement in the convective cooling effect.
The most common and most reliable method for cooling the blade trailing edges of at least air-cooled gas turbines has hitherto been trailing-edge blow-out which is very closely akin to film cooling and is often somewhat inaccurately also included in this collective term. In this method, coolant flows out through a number of orifices in the trailing edge and at the same time absorbs heat from the material of the blade. Introducing the blow-out orifices into the trailing edge gives rise to convection through the narrow cooling gap, in such a way that abovementioned displacement phenomena do not occur. On the other hand, the center distance between two cooling orifices at the trailing edge should be very small and, if possible, not exceed eight hydraulic diameters of the orifices. This design directive ensures that, when conventional blade materials are used, the temperature variation along the blade trailing edge remains within an acceptable scope, and local overheating phenomena are avoided. Thus, in the blow-out orifices taken as a whole, trailing-edge blow-out affords a large flow cross-section and contributes considerably to the consumption of coolant which it is expedient to minimize with a view to an increase in efficiency.
The situation thus arises where special fluidic conditions inside a turbine blade restrict the use of convective cooling methods for trailing-edge cooling. By contrast, because of the coolant consumption, trailing-edge blow-out has adverse effects on the efficiency of the gas turbine circulation process.
DE 196 54 115 therefore proposes to dispense with trailing-edge blow-out in the cooling of blade trailing edges and, instead, to introduce thermally highly conductive pins into the trailing-edge material, said pins projecting into the blade interior through which coolant flows. Consequently, heat is to be transported out of the trailing-edge material and transferred to the cooling-air flow. In this case, the pins, when cooled sufficiently by the coolant, act as heat sinks in the basic material of the blade. Here, too, however, there is again the problem, already discussed, that the flow boundary layers coalesce in the region of the trailing edge at the inner walls which taper at an acute angle, and the cooling flow is displaced into other regions of the blade interior. The pins further increase the obstruction of the cooling duct, and ultimately the cooling air does not flow around them to the desired extent, so that transmission of heat from the pins to the cooling medium is restricted and a suboptimal heat-sink effect of the pins is obtained as a result.